The present invention relates to chopper amplifier circuits for the supply of a current proportional to a voltage and, in particular, it applies to such circuits designed to provide deflection coils of cathode-ray tubes with the currents needed for their operation.
Hereinafter, the circuits referred to shall be mainly amplifier circuits supplying deflection coils in order to carry out a stroke deflection on the screen of a cathode-ray tube, namely a scanning where the beam can travel across the screen in any way. The current supply to the deflection coils then raises crucial problems of energy dissipation. However, the circuit according to the invention can be applied in any field where high power has to be supplied to any load.
Since a deflection coil can be likened to an inductor, the voltage at its terminals has the form L.dI/dt where L represents the inductance of the coil and dI/dt represents the derivative with respect to time t of the current I that flows through the coil. Since the shifting of the beam on the screen is appreciably proportional to I, the term dI/dt may be identified with the speed of the trace on the screen.
In a stroke deflection, when the image to be traced on the screen is charged, namely filled with details, the tracing speed should be high and hence the supply voltages should be relatively high.
Furthermore, the deflection energy needed is of the order of one millijoule. This implies deflection currents of some amperes.
It can thus be seen that the power needed for the working of a random type electromagnetic deflection system is high. Furthermore, since the deflector is reactive, almost all the power consumed is dissipated in the final stage of the amplifier.
As a consequence, it is frequently necessary to dissipate several tens of Watts in the output transistors, and this raises major problems in compact displays.
A known way to achieve this end is through the complementary association, in one and the same chopper amplifier circuit, of a linear amplifier and a chopper amplifier. The chopper amplifier circuit for the supply, to a load, of an output current proportional to an input voltage, then comprises a linear amplifier and a chopper amplifier, the output of which is coupled to the load. The linear amplifier is a power amplifier controlled to supply a current as a function of the difference between the input voltage and a voltage proportional to the output current, and its output is coupled to the load, so that the current that it supplies constitutes a part of the output current. The other part of the output current is given by the chopper amplifier which is controlled as a function of the current given by the amplifier.
In the known chopper amplifier circuits, the control of the chopper amplifier is not done satisfactorily, and the result thereof is low efficiency and difficulties in making these circuits so that they can work at high frequencies.